Question

6.00SC - PS4 - Problem 1 - When I shift "T" 8 places, I get "B", they get "A". So when I shift 'This is a test.' 8 places, I get 'Bpq hq hiham a.', they get 'Apq hq hiham a.' Who's right?

Answer 1

http://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-00sc-introduction-to-computer-science-and-programming-spring-2011/unit-2/lecture-10-hashing-and-classes/MIT6_00SCS11_ps4.pdf

They say:
```xml
In this problem set, we will use a variant of the standard Caesar cipher where the space character
is included in the shifts: space is treated as the letter after "z", so with a key of 2, "y" would
become " ", "z" would become "a", and " " would become "b".
```
So you have to take into account the added space character

Answer 2

So I ran their solution set using a shift of +1 to see how it handled both "z" and "Z". Both shifted to the space character, leading me to believe that decoding the space character would most likely always lead to a "z", never a "Z". How do you get back a "Z"? For example, San Diego Zoo would get decoded back to San Diego zoo? Maybe a better question would be: is ASCII "spacebar" different from ASCII "Shift-spacebar"?

Answer 3

What do you mean by 'Shift-spacebar'?
And from what I see they treat upper case and lower case characters the same.. so maybe it doesn't matter?

Answer 4

I'll say it better.. From what I see they use space both after 'z' and 'Z', so you can't really tell.. maybe it doesn't matter?

Answer 5

If by 'Shift spacebar' you mean if there is a 'capital' space.. then no I'm afraid not
Space is just ascii 32: http://ascii.cl/
You could theoretically use a different character (perhaps another whitespace, like tab) for upper case, but it's not what they say

Answer 6

It does matter. I encoded 'San Diego Zoo' with a +1 shift and got 'TboaEjfhpa pp'. I then decoded 'TboaEjfhpa pp' with a -1 shift and it resulted in 'San Diego zoo'.

Answer 7

Ok, so is it all good now?

Answer 8

No, you can't get back any capital letter that gets encoded to a space.

Answer 9

'San Diego Zoo' with a +8 shift will ultimately come back as 'san Diego Zoo', because the "S" gets encoded to a space at +8.

Answer 10

```python
>>> build_decoder(3)
{' ': 'x', 'A': 'Y', 'C': ' ', 'B': 'Z', 'E': 'B', 'D': 'A', 'G': 'D',
'F': 'C', 'I': 'F', 'H': 'E', 'K': 'H', 'J': 'G', 'M': 'J', 'L': 'I',
'O': 'L', 'N': 'K', 'Q': 'N', 'P': 'M', 'S': 'P', 'R': 'O', 'U': 'R',
'T': 'Q', 'W': 'T', 'V': 'S', 'Y': 'V', 'X': 'U', 'Z': 'W', 'a': 'y',
'c': ' ', 'b': 'z', 'e': 'b', 'd': 'a', 'g': 'd', 'f': 'c', 'i': 'f',
'h': 'e', 'k': 'h', 'j': 'g', 'm': 'j', 'l': 'i', 'o': 'l', 'n': 'k',
'q': 'n', 'p': 'm', 's': 'p', 'r': 'o', 'u': 'r', 't': 'q', 'w': 't',
'v': 's', 'y': 'v', 'x': 'u', 'z': 'w'}
```
They decode here ' ' in to lower case 'x'. That is what expected from you
Or are you just curios how to solve this problem hypothetically?

Answer 11

Yes, that is correct, but hypothetically, if your word was 'X-ray', the encoder at +3 would make it ' -uda', then the decoder would have it come back as 'x-ray'. You lost the capital "X". I was asking this because if I was using my script, the fable in Problem 5 most likely won't decode properly. I have changed my code to add the space in the upper case alphabet. I will end this by saying that the logic seems to be flawed. That you for helping me understand that there is not an upper case ASCII 'space' character.

Answer 12

Ye this is flawed.. no doubt about that. Hypothetically they should have used different characters for upper case and lower case, but oh well
Glad I could help =)